1. Field of the Invention
The present invention relates in general to an apparatus for sensing a small object in a high-frequency induction heating cooker, and more particularly to such an apparatus for sensing rapidly presence of the small object, such as a spoon, on a turntable of the high-frequency induction heating cooker.
2. Description of the Prior Art
Referring to FIG. 1, there is shown a circuit diagram of a conventional apparatus for sensing a small object in a high-frequency induction heating cooker. As shown in this drawing, the conventional small object sensing apparatus comprises a rectifying/smoothing circuit 1 for rectifying and smoothing an alternating current (AC) voltage from a power source to convert it into a direct current (DC) voltage and applying the converted DC voltage to an inverter 2.
The inverter 2 is adapted to convert the DC voltage from the rectifying/smoothing circuit 1 into a high frequency of AC voltage under control of a controller 4.
The conventional small object sensing apparatus also comprises a heating coil 3 for generating an induced current according to the AC voltage from the inverter 2 to heat a target object in a cooking container, a synchronization detector 12 for detecting a synchronization signal resulting from a resonance operation of the inverter 2 and outputting the detected synchronization signal to the controller 4, and a temperature detector 5 for detecting a temperature of the high-frequency induction heating cooker and outputting the resultant signal to a reset circuit 6.
The reset circuit 6 is adapted to reset the controller 4 in response to an output signal from the temperature detector 5 to stop the operation of the high-frequency induction heating cooker.
The controller 4 is adapted to control the inverter 2 in response to the synchronization signal from the synchronization detector 12 and a reset signal from the reset circuit 6.
The operation of the conventional small object sensing apparatus with the above-mentioned construction will hereinafter be described with reference to FIG. 1 and FIGS. 2A to 2D, in which FIG. 2A is a timing diagram of a voltage generated in a secondary coil L3 of a transformer T by the resonance operation of the inverter 2, FIG. 2B is a timing diagram of the synchronization signal from the synchronization detector 12, FIG. 2C is a timing diagram of a control signal from the controller 4 and FIG. 2D is a timing diagram of the output signal from the temperature detector 5.
First, the AC voltage from the power source is supplied to the rectifying/smoothing circuit 1. In the rectifying/smoothing circuit 1, the supplied AC voltage is full wave-rectified by a bridge diode BD1 and then charged on a capacitor C1. At this time, a ripple effect appears as the charging and discharging of the capacitor C1 are repeatedly performed. Such a ripple effect is removed by a choke coil L1 which connected between an output terminal of the bridge diode BD1 and a positive terminal of the capacitor C1. As a result, the smoothed DC voltage is outputted from the rectifying/smoothing circuit 1.
The smoothed DC voltage from the rectifying/smoothing circuit 1 is applied to the inverter 2. In the inverter 2, the smoothed DC voltage from the rectifying/smoothing circuit 1 is applied to a primary coil L2 of the transformer T and, thus, the AC voltage of the high frequency is induced in the secondary coil L3 of the transformer T according to a switching operation of a switching device Q1 under the control of the controller 4. The induced AC voltage in the secondary coil L3 of the transformer T results in the generation of the induced current in the heating coil 3. As a result, the target object in the cooking container is heated based on the induced current of the heating coil 3.
Namely, the induced voltage as shown by the reference numeral S1 in FIG. 2A is generated in the secondary coil L3 of the transformer T by the resonance operation of a capacitor C2 and the primary coil L2 of the transformer T in the inverter 2. Then, the synchronization detector 12 detects the synchronization signal as shown in FIG. 2B by detecting a zero crossing point of the voltage S1 induced in the secondary coil L3 of the transformer T. The detected synchronization signal from the synchronization detector 12 is applied to the controller 4. The controller 4 outputs the control signal as shown in FIG. 2C to the switching device Q1 in response to the synchronization signal from the synchronization detector 12. As shown in FIG. 2C, the control signal from the controller 4 goes high at a falling edge of the synchronization signal from the synchronization detector 12, whereas low at a rising edge of the synchronization signal from the synchronization detector 12. The switching device Q1 is turned on in response to the high control signal from the controller 4. After the lapse of a predetermined time period, the switching device Q1 is turned off in response to the low control signal from the controller 4. Also in the inverter 2, a diode D1 functions to protect the switching device Q1 against an overvoltage.
On the other hand, as the cooking operation of the high-frequency induction heating cooker is advanced, temperatures of the bridge diode BD1 and the switching device Q1 are increased, resulting in an increase in a resistance of a thermistor TH1 in the temperature detector 5. In the temperature detector 5, a reference voltage Vcc appears mostly at a resistor R1 and the thermistor TH1, whereas little at a resistor R2, with the increase in the resistance of the thermistor TH1. As a result, the output signal from the temperature detector 5 is changed from a high level to a low level as shown in FIG. 2D. The output signal from the temperature detector 5 as shown in FIG. 2D is applied to the reset circuit 6, which thus outputs the reset signal to the controller 4.
Assuming that the small object such as a spoon or a metallic bit is placed on a turntable of the high-frequency induction heating cooker, a small amount of current flows through the switching device Q1 and a high voltage is generated therein, resulting in an abrupt increase in the temperature. The abrupt increase in the temperature of the switching device Q1 increases the resistance of the thermistor TH1 in the temperature detector 5, thereby causing the temperature detector 5 to output the low signal to the reset circuit 6, as mentioned above. As a result, the high-frequency induction heating cooker is reset in response to the reset signal from the reset circuit 6.
However, the above-mentioned conventional small object sensing apparatus has a disadvantage in that it takes the system temperature much time to rise to a predetermined value for the sensing of the small object, because the small object is sensed on the basis of the increase in the temperatures of the bridge diode and the switching device. Also, it takes the system much time to be restored to a normal state after the small object is removed, since the system is released from the reset state only when the system temperature falls below the predetermined value.